Loopback testing is performed by voice over internet protocol (“VoIP”) service providers, which are often cable television network operators (sometimes referred to as multiple services operators (“MSO”)) to test the connectivity and quality of connection from a network test point and their subscribers' network interface units (“NIU”). ‘Connectivity’ refers to the ability to establish a voice call with a telephony subscriber and ‘quality of connection’ refers to levels of jitter, latency, and packet loss for packets, and voice path transmission characteristics such as idle channel noise. MSOs typically use two different types of loopback: real time protocol (“RTP”) packet loopback and pulse code modulation (“PCM”) loopback. MSOs use RTP loopback to measure the quality of connection at the packet level and PCM loopback to measure quality of connection at the PCM level of a voice call.
Loopback allows MSOs to determine call quality based on returning media to the originator of a call. Typically, MSOs implement loopback based on an extension to the Session Description Protocol for Media Loopback (draft-ietf-mmusic-media-loopback-05). MSOs typically support two types of loopback: media loopback and packet loopback. As with NETWTEST according to the PacketCable specification, media loopback passes through the DSP on the mirroring side. On the other hand, packet loopback re-encapsulates RTP data in IP, UDP, and RTP headers. This is analogous to NETWLOOP under PacketCable specification.
When a communication device, such as an IP telephone, or other communication device that carries VoIP calls, receives a loopback call, the receiving device does not ring, but a media terminal in the device typically ‘answers’ the call and processes the loopback operations. The OPTIONS method may be used to query the MTA to determine if a connection is available for loopback. In addition, the OPTIONS method may define a services delivery platform along with the codecs the platform supports, thus allowing an MSO to determine if a queried MTA supports loopback.